1. Technical Field
The invention relates to noninvasive spectroscopy. More particularly, the invention relates to the modeling of human tissue for use in noninvasive spectroscopy.
2. Description of the Prior Art
In the field of noninvasive spectroscopy, photons generated by a source penetrate into the body of a subject, interact with the subject""s tissue layers and exit to a detector. The interaction with the tissue layers is complex and is not well understood. Models that simulate the tissue may be utilized to address such fundamental questions as the net analyte signal, depth of penetration of the photons and radial diffusion of the photons. Knowledge of the exact chemical composition of a tissue surrogate will allow chemical and physical interpretation of spectra obtained on human skin where the exact chemical composition of the sample is unknown. For these reasons, a model of skin tissue samples would be beneficial.
The invention provides a class of samples that model the human body. This family of samples is based upon emulsions of oil in water with emulsifiers such as lecithin used to keep the solution from separating. These emulsions have oil droplets with varying particle sizes acting as scatterers and may be spiked with basis set components (e.g. albumin, globulin, urea and glucose) to simulate skin tissues further. The family of samples is such that other organic compounds, such as collagen, elastin, globulin, lactic acid and bilirubin may be added, as can salts such as Na+, K+ and Clxe2x88x92. Layers of varying thickness with known index of refraction and particle size distributions may be generated using simple crosslinking reagents, such as collagen. The resulting samples are flexible in that each analyte""s concentration may be adjusted independently of the others and that each skin layer of the body may be matched in terms of the samples"" reduced scattering and absorption coefficients, xcexcxe2x80x2s and xcexca.
Physiological glucose concentrations are determined in diffuse reflectance mode using near-IR spectroscopy on novel tissue-simulating phantoms. The tissue phantom, which is composed of water and a modified Intralipid solution is similar to skin of the human forearm in terms of its absorption and reduced scattering coefficients. Albumin and urea are added to the samples acting as additional interferences present in the body and as diluents allowing experimental designs that ensures that the glucose concentration is not correlated with time, additional matrix constituents or reference spectra. All major near-IR absorbers of skin tissue in the 1100 to 2500 nm region are present in the resulting tissue phantom. Using reference spectra to model instrumentation drift, an f-test demonstrates that multivariate analyses are not modeling correlations between glucose concentrations and spectrometer variations. Glucose determinations are demonstrated independently in the 2nd overtone region, 1st overtone region and combination band region with SEP""s of 40.0, 13.5 and 29.6 mg/dL, respectively. This work demonstrates the feasibility of diffuse reflectance near-IR determination of glucose in the body.
This family of samples is provided for use in the medical field where lasers, laser diodes, LED""s and spectroscopy based analyzers are used in the treatment of the body. In particular, knowledge may be gained on photon depth of penetration, photon radial diffusion, photon interaction between tissue layers, photon density (all as a function of frequency), and on instrument parameter specifications, such as resolution and required dynamic range (i.e. A/D bits required). In particular, applications to delineate said parameters have been developed for the application of noninvasive glucose determination in the near-IR region from 700 to 2500 nm with an emphasis on the region from 1000 to 2500 nm (10,000 to 4000 cmxe2x88x921).